cmd/oci-image-tool/validate.go: create a logger if none exists.
cmd/oci-image-tool/validate.go: create a logger if none exists.

Signed-off-by: Erik Hollensbe <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@hollensbe.org>

opencontainers/image-tools,opencontainers/image-tools